Incandescent cathode for electric discharge tubes



March 3, 1931. A. E. VAN A-RKEL ET AL INCANDESCENT CATHODE FOR ELECTRIC DISCHARGE TUBES Filed May 14, 1924 MuWmm /MM Maw Patented Mar. 3, 1931 UNITED STATES PATENT OFFICE ANTON EDUARI) VAN ARKEL AND J'OHANNES BIRIUIIIJN'ES, OF EINDHOVEN, NETHER- LANDS, ASSIGNORS TO NAAMLOOZE VENNOOTSCHAP PHILIPS GLOEILAMPEN-jw- FABRIEKEN, OF EINDHOVEN, NETHERLANDS INCANDESCENT CATHODE FOR ELECTRIC DISCHARGE TUBES Application filed May 14, 1924, Serial No. 718,223, and in the Netherlands May 18, 1928.

This invention has reference to incandescent cathodes for electric discharge tubes such as rectifiers with or without gaseous filling; receiving or transmission tubes for wireless telegraphy, telephony or similar purposes, with three or more electrodes and with or without gaseous filling; X-ray tubes; are lamps with enclosed arc discharge; etc.

The invention has for its object to choose such material for incandescent cathodes that the emission of electrons can be obtained with less energy than in the case of the material usually used in practice for such electrodes, that is to say tungsten.

For that same purpose it has already been suggested to use electrodes containing thorium.

According to the invention the incandescent cathode contains at its surface titanium 2Q nitride or zirconium nitride or hafnium nitride or a mixture of two or more of these compounds.

The said compounds yield a greater emission of electrons than tungsten which is perceptible even when these compounds constitute part of the surface of the electrode. This characteristic stands out most clearly when the surface of the electrode consists entirely of one or more of the said compounds.

For the emission of electrons it is essential that the said compounds be found on the surface of the electrode but it is not necessary that the entire electrode should consist of the said compounds. On the contrary it is often preferred in practice to apply one or more of the said compounds to a body consisting of some other suitable material.

According to the invention this body may consist of an highly refractory material which is a goo conductor of electricity, preferably a highly refactory metal such as platinpm or molybdenum or tungsten or'a metal a 0y.

It may be advantageous to use as a core those metals or metal alloys having a coefficient of expansion corresponding to that of the compound applied, for example platinum rhodium in the case of zirconium nitride.

First of all, the melting point of the said core material should lie above the temperature favorable to the emission of electrons, which for example in the case of zirconium nitride lies near about 1000 C., but in addition it may be required for the manufacture 'conium nitride or hafnium nitride to any body. 1 n

In the process according to the invention a body is heated in an atmosphere containing a reducing substance, nitrogen, and one or more dissociable compounds of titanium or Zirconium or hafnium in a state of vapor, to such a temperature that on its surface titanium nitride or zirconium nitride or hafnium nitride or a mixture of these compounds is precipitated.

As dissociable and volatile compounds of titanium, zirconium or hafnium the halogenides of these elements, preferably the chlorides, may be used.-

The reducing substance may for example consist of an alkaline vapor, but preferably it consists of carefully dried hydrogen.

An example of the process according to the invention will be described in detail with reference to the accompanying drawing which diagrammatically illustrates apparatus suitable for carrying it out. I

A mixture of nitrogen and hydrogen is delivered through a glass tube 1. The content 2 where it is strongly cooled. since the vessel '2 is surrounded by a vessel 3 provided-with liquid air. The mixture then flows through the tube 4 to the vessel 5 in which the substance 6 is enclosed.

If it is desired to precipitate zirconium nitride, the said substance 6 is preferably zirconium chloride and the vessel 5 should be maintained at such a temperature that the chloride evaporates to a sufficient extent.

'ihe gas mixture containing the zirconium chloride vapor flows from the vessel 5 through the tube 7 to the vessel 8 in which the tube 7 is provided with a number of apertures so that the gas mixture flows from the tube 7 across the wire 12 to the' apertures in the tube 13.

The tube 7 as well as the vessel 8 should be maintained at such a temperature that the zirconium chloride remains in a state of vapor. The vessels 5 and 8 and the tube 7 may be heated for example to a temperature of about 150 to 300 C.

The vessel 8 is hermetically sealed at the top by a stopper 9, for example of glass, in which the leading-in wires 10 and 11 are sealed. The wire 12 is in conductive connection with the wires 10 and 11, for example by means of small screws, so as to enable it to be heated electrically $0 a suitable temperature.

For practical purposes this temperature may be about 1800 C. However, a coating of zirconium nitride is produced on the wire at 1000 C. though at a much lower speed than at a higher. temperature.

As a rule the speed of reaction is found to increase as the temperature is raised. The temperature should not be increased too much otherwise the nitride coating grows on the wire so rapidly that a good coherent layer cannot be therefor obtained. In general it seems undesirable to increase the temperature over about 2000 C.

The choice of the material for the wire 12 depends on not only its final utilization but also on the temperature which is to be maintained for the reaction. If 1000 C. is con sidered sufiicient, nickel for example may be used. It is more advantageous to use for example tungsten or molybdenum or platinum at higher temperatures.

ture, by reason of the high temperature which prevails near the wire 12, is associated with the chlorine of thezirconium chloride to form hydrochloric acid, whereas the nitrogen resent is associated with the zirconium to i orm zirconium nitride which is precipitated on the wire. The gas mixture contaminated by hydro'chloric acid flows through the tube 13 to the vessel 14 where it is strongly cooled as the vessel 14 is surrounded by a vessel 15 containing liquid air. The remaining zirconium chloride and the hydrochloric acid are deposited in the vessel 14 and the residue of nitrogen and hydrogen leaves the apparatus through the tube 16. The zirconium chloride enclosed in the discharge tube.

In the vessel 8 the.hydrogen of the gas mix-- collected in 14 may be used again after being purified from hydrochloric acid.

The reaction is continued until the precipitate on the wire 12 has obtained the desired thickness, which can be easily ascertained by measuring for example the change of resistance of the wire 12.

The treated wire 12 which therefore consists for example of a tungsten core with a precipitate of zirconium nitride is suitable for use as an incandescent cathode in a discharge tube.

It is advisable to age the wire when it is The emis sion of electrons of the wire increases thereby until a given invariable value has been 1 reached; The incandescent cathode thus obtainedhas very important advantages. The emission of electrons is very high. At about 900-1000 C. the emission is altogether sufficient for example for receiving tubes for wireless ielegraphy although the vapor tension is still imperceptible. It is for example possible to obtain a current of electrons of about five milliampers by supplying only an energy of one watt to the incandescent wire.

Once the electrode is prepared by aging, the emission of electrons remains invar1ab e for a long time. It has for example been ascertained that after 1000 hours of service the emission was still practically the same as at. the beginning. The coating of zirconium nitride adheres well to the core and does not crumble ofi.

It is also important to notice that the coating of zirconium nitride is a very good conductor and that the wire can be welded to nickel or to any other suitable material without any difiiculty. It is of course obvious that this is of great convenience in fixing the incandescent wire to the supporting wire.

In a manner similar to that described for zirconium nitride, it is possible to make an incandescent cathode coated at its surface with titanium nitride. In this case titanium chloride (TiCl is used as the dissociable compound, said titanium chloride boiling at 140 C., so that the vapor pressure at ordinary temperatures is high enough for the reaction to take place, and it is therefore superfluous to heat the vessels 5 and 8. The wire 12 is to be heated to a temperature that may vary from 1000 to 2000.C. The TiCl is partly reduced by the hydrogen into TiCl which is slightly volatile and is precipitated on the wall of the vessel '8. About the titaniumnitridethefollowingmaybestated. The

The emission has been found to have a considerable value at a temperature of about 1600C. I a Incandescent wires according to the invention' more particularly those containing zir-.

conium nitride may be used to advantage for example in small receiving tubes with three or more electrodes such as are used by amatours for radiotelegraphic purposes. The incandescent cathode consumes only onefourth to one-fifth of the energy required by a tungsten cathode and this is of importance particularly for the amateurs.

The incandescent cathode according to the invention may render good services in X-ray tubes and more particularly in high vacuum X-ray tubes.

In arc lamps with enclosed arc discharge an incandescent cathode according to the invention' may be arranged opposite an anode of for example tungsten; the light is produced principally by the latter.

What we claim is:

, 1. A cathode for electric dischar e tubes comprising a metallic core and a sur ace consistingof -a nitride of at leastone of the group of elements comprising titanium, zircon1um andhafnium.

8. A process for making a filament of the character described, comprising vaporizing at least one of the chlorides of the group of In testimony whereof we afiix our signatures, at the city of Eindhoven, this 22nd day of April, 1924.

ANTON EDUARD VAN ARKEL. JOHANNES BRUIJNES.

2. A cathode for electric discharge tubes,

characterized in that it consists of a highlyrefractory material which is a good conductor of electricity, said cathode being coated with a nitride of at least one of the group of elements comprising titanium, zirconium and hafnium.

3. A cathode for electric discharge tubes, characterized in that it consists o a body of at least one hi hly refractory metallic material, said body eing coated with a compound of at least one of the group of elements comprising'titanium, zlrcomum and hafnium.

4. A cathode for electric discharge tubes characterized in that it consists of a core 0 at least one highly refractory metallic material, covered with a coating that principally consists of zirconium-nitride.

5. A process for making a filament of the character hereinbefore described, which includes the step of heating a wire in a,fluid comprising a volatilized compound of a metal, a reducing agent for reducing the com pound and an element for combinin with the liberated metal from the compoun 6. A process for making a filament of the.

character hereinbefore described, which includes the step of heating a wire in a fluid comprising a volatilized compound of at least one of the compounds of the and hafnium a reducing agent for reducing the compound and an element for combinigg with the liberated metal from the redu compound.

' 7. A process for making a filament of the cup of elements comprising titanium, .z1rcomum 

